Coiled tubing operations may be employed at an oilfield to deliver a downhole tool to an operation site for a variety of well intervention applications such as well stimulation, the forming of perforations, or the clean-out of debris from within the well. Coiled tubing operations are particularly adept at providing access to highly deviated or tortuous wells where gravity alone fails to provide access to all regions of the wells. During a coiled tubing operation, a spool of pipe (i.e., a coiled tubing) with a downhole tool at the end thereof is slowly straightened and forcibly pushed into the well. For example, a clean out tool may be delivered to a clean out site within the well in this manner to clean out sand or other undesirable debris thereat.
Unfortunately, the coiled tubing is susceptible to helical buckling as it is pushed deeper and deeper into the well. That is, depending on the degree of tortuousness and the well depth traversed, the coiled tubing will eventually buckle against the well wall and begin to take on the character of a helical spring. In such circumstances, continued downhole pushing on the coiled tubing simply lodges it more firmly into the well wall ensuring its immobilization (i.e. coiled tubing “lock-up”) and potentially damaging the coiled tubing itself. This has become a more significant matter over the years as the number of deviated extended reach wells has become more prevalent. Thus, in order to extend the reach of the coiled tubing, a tractor may be incorporated into a downhole portion thereof for pulling the coiled tubing deeper into the well.
For a conventional tractor, power may be supplied downhole by way of an electric cable. However, unless the inner diameter of the coiled tubing is undesirably increased, this reduces the available internal flow space of the coiled tubing. Alternatively, the electric cable may be integrated into the wall of the coiled tubing. However, as a practical matter, this presents significant manufacturing challenges. In either case, the addition of electric cabling to the coiled tubing assembly may significantly increase the total weight thereof. This added load may play a significant role in the total achievable depth of the coiled tubing. Furthermore, given the independent nature of the coiled tubing and electric cable, other challenges may be presented in terms of the effort required to maintain compatible tension and control simultaneously through such separate lines of equipment.
In light of the complications presented by the incorporation of an electric cable, the tractor may be hydraulically driven as detailed in U.S. application Ser. No. 11/772,181 entitled Hydraulically Driven Tractor, incorporated herein by reference in its entirety as noted above. That is, given the availability of hydraulics provided via coiled tubing, the use of an electric cable may be avoided in certain circumstances. As such, avoiding space issues and other complications as noted above may be appreciated.
Unfortunately, regardless of the manner of powering the tractor, the overall reach of coiled tubing in a well remains limited. This is due in large part to the fact that the load carrying capacity of any given tractor faces its own limitations. For example, in the case of a cable powered tractor, sending more power downhole will eventually result in damaging of the tractor as opposed to extending the reach of the coiled tubing assembly. The hydraulically powered tractor on the other hand avails itself the possibility of either increasing pressure or reducing flow rate in order to increase overall load carrying capacity. Nevertheless, at some point increased pressure also results in tractor damage while slowing down of the flow rate slows the speed of the operation and may even halt it altogether. Overall, the load carrying capacity of a tractor in a conventionally sized well may be limited, for example, in many cases to no more than between about 5,000 lbs and 7,500 lbs.
At present, wells of ever increasing depth and deviation are being employed, often well beyond 10,000 feet in depth, and often in need of interventional operations that traditionally lend themselves to the utilization of coiled tubing. However, given the above noted load limitations of available tractor assemblies, the extent of the reach of the coiled tubing may be affected by a host of factors related to the overall load, such as the degree of deviated character of the well and the increasing weight of the coiled tubing assembly as it is advanced further and further into the well. Generally speaking, a conventional tractor driven operation employing standard coiled tubing (i.e. coiled tubing that is between about 2″ and about 3½″ in diameter) may have an effective reach of less than about 10,000 feet in a highly deviated well.